Bill validators used in vending machines and the like typically utilize various styles of reflective optical sensors to obtain measurements from an inserted bill to determine authenticity, denomination and location. Typically, the bill is transported past at least one photosensor, having a light-emitting diode (LED) and photodetector (photodiode or phototransistor).
Some factors that adversely affect the bill measurements include the following: inserted bills are of different denominations, cleanliness and quality; bill may be creased or crumpled, and the bill location and inclination across passageway may strongly vary. In addition, the output power of LED can vary due to age and/or ambient conditions. Furthermore, there are normal production variations in LED optical power output and detector sensitivity, which can lead to sensors having varying current and voltage requirements in order to operate effectively. In order to partially offset these factors, optical sensor measurements are taken over a large dynamic range. As power of LED and sensitivity of photodetector are limited, the optical efficiency should be high to improve the performance of the sensors.
In the art, many embodiments of reflective optical sensors are known. The simple sensors comprise at least one photo emitter and one photo detector with relatively wide spatial diagrams (U.S. Pat. Nos. 4,348,656; 4,628,194; 5,222,584; 5,476,169; 5,692,067; 5,751,840; 5,855,268; 5,889,883; 5,909,503; 5,960,103). Such sensors have low optical efficiency and their output signal strongly depends on bill location and inclination across passageway. The space required to mount the sensors (footprint) slightly exceeds the total area of the emitters and detectors.
To improve optical efficiency, many sensors mount the emitters and detectors at an angle to one another and converging on the bill surface (U.S. Pat. Nos. 4,041,456; 4,628,194; 4,973,851; 5,420,406; 5,467,405; 5,483,069; 5,918,960; 5,992,601; 6,028,951; 6,073,744). These sensors require special optical heads, receptacles etc. The footprint for these sensors significantly exceeds the total area of emitters and detectors due to the various mounting and carrying paths. Even with this more complicated design, the output signal from these sensors strongly depends on bill location and inclination across passageway.
Advanced sensors in addition to plurality of LED's and photo detectors comprise various focusing, light guiding and reflecting elements, including fiber optic “fish tails” and splitters (U.S. Pat. Nos. 5,308,992; 5,381,019; 5,616,915; 6,044,952; 6,104,036; 6,163,036; 6,188,080; 6,359,287; 6,392,863). These sensors are more complicated, large and expensive, require special optical parts and often require additional alignment during validator assembly. The output signal of these advanced sensors continues to be largely dependent on bill location and inclination across passageway.
Some special optical sensors conduct bill scanning by means of LED's and detectors arrays with special lenses or by direct TV image or light beam scanning (U.S. Pat. Nos. 4,179,685; 4,197,584; 4,293,776; 6,363,164). This technology is expensive and is not suitable for mass production and utilization.
Some optical shadow on a bill may occur with the majority of prior art sensors because of bill inclination, illumination or observation.
It is a general object of the present invention to provide a simple reflective space efficient sensor having high optical efficiency for bill examination and other applications.
The present invention overcomes a number of the disadvantages described above with respect to the prior art sensors.